Powder metal parts are made by first compacting a metal powder composition into a desired shape in a die to produce a green metal part. The green metal part is then sintered at an elevated temperature to produce the powder metal part. Powder metal parts offer the advantage of being able to be formed into parts having intricate designs that frequently cannot be made by casting, forging or machining. In such applications, powder metal parts can typically be made at a reduced cost as compared to parts of similar design wherein machining is required. Accordingly, for economic and practical reasons, powdered metal parts are commonly used in a wide variety of applications.
Copper is frequently added to the metal powder utilized in making powder metal parts to promote rapid hardening during the cooling step as the powdered metal part is cooled after being sintered. However, the presence of copper in the metal composition causes the part to expand during sintering and leads to a significant variation in the tolerance of parts produced. In other words, the presence of copper makes it difficult to control the size and uniformity of such powder metal parts. This problem becomes more pronounced in the case of larger parts. For instance, it is typical for powder metal parts containing copper to expand from 0.002 to 0.005 inches per inch of part diameter.
U.S. Pat. No. 3,837,845 discloses a steel powder particularly adapted for use in powder forging and having a composition consisting of about 0.05-0.15% carbon, about 0.8-2.5% silicon, up to less than 1% manganese, about 0.5-4% nickel, about 0.2-2% molybdenum, up to about 0.2% columbium, up to 2% copper, about 0.01-0.25% oxygen, the balance being essentially iron (see claim 1).
U.S. Pat. No. 4,363,662 discloses an abrasion resistant ferro-based sintered alloy comprising 1.1-1.6% carbon, 1.5-3.5% chromium, 1.6-2.9% molybdenum, 1.0-3.0% nickel, 3.0-5.0% cobalt, 0.5-1.5% tungsten, 1.8-18.0% copper and balance iron (see claim 1)
U.S. Pat. No. 4,803,409 discloses an alloy steel powder for powder metal applications, the powder including 0.2-2.0% tungsten, 0.8-3.0% nickel, 0.2-2.0% copper, and the balance substantially iron (see col. 2, lines 30-34).
There is a need for a technique for manufacturing hardened powder metal parts that are uniform and of consistent shape. However, heretofore the inclusion of copper in such parts to attain desired levels of hardness has not been possible without compromising part tolerances and uniformity. In the case of many parts such a compromise cannot be tolerated.